The Use of Devices for Measuring Fluid Movement Conditions to Improve the Operating Efficiency and Reduce the Maintenance Cost of Moving-Fluid-Driven Working Devices
Various devices (called “Working Devices” herein; see below for definitions of this and other terms) are used to derive useful work from moving fluid streams. An example of such a Working Device is a wind turbine generator (called “WTG” herein), which is often configured with one or more blades attached to a rotating hub, with the blades configured so as to create aerodynamic lift in the moving fluid stream, causing the hub to rotate and deliver torque to operate an electric generator. In order to operate efficiently, the Working Device must be oriented optimally and must have its internal components configured optimally with respect to the fluid stream. In the case of a WTG, this implies that the axis of symmetry of the rotor must be oriented in a horizontal plane as directly as possible into the air stream and the blades must be rotated around their long axis so as to optimize the energy capture through lift and so as to mitigate the stress on the WTG components. If these are not accomplished, the WTG may “spill” usable energy in the airstream and may subject the device to excessive stress, causing premature wear and potential component failure.
In response to this need, various devices (called “Measuring Devices” herein) are used to measure fluid movement conditions (velocity, direction, turbulence, etc.) at the location of the Measuring Device. An example of such a Measuring Device is an anemometer, used to measure movement conditions in the airstream. In the case of a WTG, the anemometer is typically mounted on top of the nacelle, downstream from the rotor. This anemometer measures wind velocity, direction, and other data at the location of the anemometer and provides these data to the WTG controller, which in turn “Yaws” the WTG (i.e., rotates the WTG in a plane approximately horizontal) to face into the wind and “pitches” the rotor blades (i.e., rotates the blades around the long axis of each blade) so as to (1) obtain as much aerodynamic lift as is possible, up to the amount of lift necessary to generate the full rated power of the generator and (2) to reduce the stress on the rotor and other WTG components.
A defect in this strategy of deploying fluid Measuring Devices is that the WTG (or other Working Device) requires some time, typically a few seconds, to respond to the data acquired by the Measuring Device and adjust the orientation and internal configuration of the Working Device, so that the Working Device is unable to respond to current conditions and always remains a few seconds out of sync with the fluid movement conditions prevailing at the Working Device. In response to this need, certain Distant Measuring Devices, such as LIDAR (Light Detection And Ranging) and SODAR (Sonic Detection And Ranging), are used to measure fluid movement conditions at a distance from the Working Device and provide these data to the Working Device Controller, enabling the Working Device to be optimized with respect to the conditions prevailing at the Working Device. These Distant Measuring Devices often function by projecting a wave phenomenon such as electromagnetic radiation or sound toward the region of the moving fluid of interest, detecting a portion of the wave which is reflected back by discontinuities in the fluid (such as dust particles if the fluid is air), and measuring the Doppler shift between the outgoing and returning waves. When combined with data such as the length or frequency of the wave, the Doppler shift can be used to determine the movement conditions of the discontinuity, from which is inferred the movement conditions of the fluid in the vicinity of the discontinuity. This application is deployed expressly for the purpose of improving operational efficiency of and reducing component wear in the Working Device (U.S. Pat. Nos. 7,281,891, 7,391,506, 7,342,323, and 6,320,272 and U.S. Patent application 20090047116). Such a Distant Measuring Device does not need to be attached to or located near the Working Device, provided that the Distant Measuring Device is located so that it can measure the moving fluid that will impinge on the Working Device and communicate with the Working Device.
The Design Process of a Wind Turbine and Similar Working Devices
The design of a wind turbine or similar Working Device typically involves a process as follows: (See FIG. 1.)                1) Preliminary design, ending in an approximate configuration of the Working Device and all components. This phase may involve various optimization studies which attempt to approximate a design of the Working Device which maximizes economic value.        2) Detailed design, ending in a precise configuration of the Working Device and precise configuration and specification of all components.        3) Aeroelastic or similar modeling (called “Fluid-elastic Modeling” herein) of the detailed design.        4) Finite element analysis (FEA) of the detailed design based on the loads predicted by the Fluid-elastic Model.        5) If the FEA shows any component to be over allowable stress, iteration of steps 2 through 5.        
Steps 3 and 4 involve exposing the Working Device virtually to a set of load conditions (typically several hundred load conditions) and predicting the device's behaviour (particularly its ability to withstand mechanical or structural load) in each load condition. Each load condition represents a set of environmental conditions, such as, in the case of a WTG, wind speed, wind direction, upflow angle, grid condition, etc., plus the rate and extent of change in each. The set of load conditions is intended to be a proxy for the full range of actual loads under which the Working Device will be expected to operate during its service lifetime (in the case of a WTG, typically 20 years or more). For each load condition, the designer (or, typically in the case of a WTG, design team) of the Working Device performs a Fluid-elastic Model and Finite Element Analysis of the Working Device, determining for every component in the Working Device (there are typically several hundred or more in a WTG) the % of allowable stress to which the component is exposed during the load condition in question in each of the possible failure modes (compression, tension, shear, moment, deflection, fatigue, etc.) The allowable stress is a function of the configuration of the component, the strength of the material, and the configuration of the Working Device as a whole. Therefore, in the case of a WTG, the designer of the Working Device will typically produce a 4n matrix in which the X-axis represents several hundred load conditions, the Z-axis represents several hundred components in the WTG, the W-axis contains the possible failure modes, and the Y-axis contains a datum equal to the % of allowable stress. In total, in the case of a WTG, there will usually be a few million such data. (This is easier to consider as a 3n matrix in which the Y-axis contains a histogram describing the % of allowable stress in the critical failure mode. Typically, FEA programs display stress in the critical failure mode graphically in a single visual display of the component.)
Every such datum, for every component and for every load condition, must fall at or below 100% of allowable stress in all failure modes. Any component which fails this test (see FIG. 2, load conditions 1 through 13) will be a potential source of serial defects in the Working Device when it is in commercial production, thereby increasing the manufacturers' warranty exposure and the customers' maintenance costs. Furthermore, wind turbines and other Working Devices are often sold with the benefit of a certification from a certifying agency (in the case of WTGs, typically either Germanischer Lloyd or Det Norse Veritas), and such agencies typically require that all components of the Working Device meet this test for a full range of stipulated load conditions.
As discussed above, the allowable stress of any component is a function of the configuration of the component, the strength of the material, and the configuration of the Working Device as a whole. For each component which, in any load condition, exceeds 100% of allowable stress, the designer of the Working Device has three options within prior art: First, the designer may change the configuration of the component, for example, by making it bigger. For example, a bolt which is over 100% of allowable tension loading under a certain load condition may fall below 100% of allowable tension loading if increased in diameter. However, adding mass to the component adds to the cost of the component and, more importantly, adds to the load on other components in the Working Device, potentially increasing their cost as well. Second, the designer may upgrade the strength of the material that comprises the component. For example, a WTG rotor blade made of fiberglass may have carbon fibers incorporated into the fabric to improve strength and stiffness. However, this also adds to the cost of the component and may adversely affect loads on other components. Third, the designer may reconfigure the Working Device as a whole to mitigate the failure. For example, if the FEA shows that a WTG rotor blade will deflect enough to engender tower strikes, the designer may increase the rotor camber (the angle of the rotor plane from vertical) to mitigate the possibility of tower strikes. However, reconfiguring the Working Device will change the load on other components and may cause other components to be overstressed.
Since such Working Devices are typically complex machines whose components interact functionally and structurally, it is usually necessary, within prior art, to repeat steps 2 through 5 very many times to eliminate all load conditions in which one or more components is subjected to stress in excess of 100% of allowable stress (called “Offending Load Conditions” herein). (FIG. 1A).
Furthermore, within prior art, there is an inherent limitation in the degree to which the design can be optimized. Consider for illustrative purposes a single component under the full range of load conditions; consider further that the data (% of allowable stress) for the range of load conditions is sorted by % of allowable stress. (See FIG. 2.) There will typically be a wide variation in the data. The component must be strong enough to withstand the most severe load it will ever experience, but this determining load condition is likely to be encountered only for a tiny fraction of the Working Device's service life. Thus, to survive for a tiny % of its operating conditions, the Working Device is likely to be much heavier and stronger, and therefore much more expensive, than it needs to be for all other operating conditions. (See FIGS. 2 and 3.)
(Note that this analysis concerns only load conditions under which the Working Device is operating and performing useful work. For example, WTGs are typically equipped with anemometers which measure wind speed in real time (i.e., as the wind passes over the WTG) and which enable the WTG controller to curtail operation of the WTG when the wind speed exceeds a certain maximum “cut-out” wind speed, typically 20 to 25 meters per second. Such curtailment is typically achieved either by feathering the blades, by braking the drive shaft, by causing the rotor to stall passively, or by some combination of these techniques. After cutting out, operation typically does not resume until the wind speed has dropped below the cut-out speed for a period of time; this loss of production is considered a “hysteretic” effect for purposes of the economic performance of the WTG.)
In view of the discussion above, a method for (1) reducing the number of iterations in the Working Device design process, (2) reducing or eliminating the cost of redesigning the Working Device, and (3) reducing the cost of manufacturing the Working Device as a result of the redesign would represent a significant advance in the art. The present invention provides such a method.